The serial size is not the depth, and inside a serial size it moves
A 305x165x40 UB is not 305mm deep. It is 303.4. That much most people know, or at least suspect, and a couple of millimetres rarely hurts anybody.
What does hurt is that the depth changes within a serial size. A 203x133x25 is 203.2mm deep. A 203x133x30, the very next section up, is 206.8mm. They share a designation and a rolling, and they are three and a half millimetres apart. Larger serial sizes spread further than that.
The reason is how the sections are made. Rolling a heavier beam in the same family does not hollow out more steel, it adds metal to the outside of the flanges. So the heavier section is deeper and wider than the lighter one, not just denser. If you have set an opening, a soffit or a floor zone out to the serial size and the engineer then goes up a weight for deflection, the beam that arrives is bigger than the hole you built.
The third number is the one that is exact
Of the three figures in a UK designation, the first two are approximate and the third is not. Mass per metre is exact, it is what the section is bought by, and it is why it is in the name at all. Depth and width are the serial family; kilograms per metre are the invoice.
That makes this a costing tool as much as a reference one. Pick the section, put in the lengths and the count, and the tonnage is what a stockholder quotes against and what decides whether it goes on a wagon or needs a crane.
The dimension people forget to look up
Depth between fillets, listed as d. It is the clear depth of web between the two root radii, and it is what decides whether something fits inside the section: a stiffener, a bolt head, a duct passing through a hole, a fitter's hand. Take the overall depth, knock off two flange thicknesses and two root radii, and there is noticeably less room than the headline number suggests. On a 203x133x30 the section is 206.8 deep and the web between fillets is 172.4.
Where these numbers come from, and what has been done to them
The dimensions and properties are from British Steel's own published datasheets to BS EN 10365:2017, which is the manufacturer that rolls them. That matters more than it sounds: section tables get copied from site to site and errors travel with them, so it is worth saying plainly where a number originated.
Every row has also been reconciled against its own geometry before being published here. The mass is recomputed from two flanges, a web and four fillets at 7850 kg/m³ and has to land within 3% of the printed figure, and the depth between fillets has to equal the depth less two flanges and two radii. That check found genuine faults during the build, including a set of wide beams whose fields had shifted by a column. One row that will not reconcile at all is left out rather than guessed, because a wrong web thickness on a beam somebody details or prices is worse than a gap.
This is a UK table and only a UK table
Two beams that are both called a universal beam, both metric, and both about 40kg a metre, six tenths of a millimetre apart in depth, and to different standards:
| Market | Designation | Overall depth | Standard |
|---|---|---|---|
| UK | 305x165x40 | 303.4mm | BS EN 10365 |
| Australia | 310UB40.4 | 304mm | AS/NZS 3679.1 |
| US and Canada | W shapes, such as W12x26 | Inches, not millimetres | ASTM A6 and CSA |
The W shape is the one that catches people twice over, because the same letter is used for two different unit systems depending on where the drawing came from.
So typing an Australian designation into the search here returns nothing, on purpose. A near miss would be the most dangerous possible answer: right units, plausible number, wrong steel.
Common questions
How deep is a 305x165x40 UB?
303.4mm, not 305. The serial size is a family label rather than a dimension. Every UK universal beam is close to its serial depth and almost none is exactly it, so setting out to the designation is a habit worth losing.
Do all sections in a serial size have the same depth?
No. This catches people: a 203x133x25 is 203.2mm deep and a 203x133x30 is 206.8mm. The extra weight goes on the outside of the flanges, so the heavier section in a serial size is both deeper and wider. An opening set out for one will not necessarily take the other.
What does the third number in a UK beam designation mean?
Mass in kilograms per metre. So a 305x165x40 is roughly 305 deep, 165 wide and 40kg/m, and the mass is the only one of the three that is exact. It is also the number steel is priced by, which is why it is in the designation at all.
How much does a steel beam weigh?
Mass per metre times length. A 305x165x40 at 6 metres is 240kg. Pick a section here, put in the length and the number of them, and it gives you the tonnage, which is what a stockholder quotes against.
What is the depth between fillets?
The clear web depth between the root radii, listed as d. It is the dimension that decides whether a stiffener, a bolt or a service fits inside the section, and it is a good deal less than the overall depth once you take off two flanges and two radii.
Can I use these for an Australian or American section?
No. An Australian 310UB40.4 is 304mm deep to AS/NZS 3679.1 and a UK 305x165x40 is 303.4mm deep to BS EN 10365. They look interchangeable and are not. American W shapes are different again. Searching an Australian designation here deliberately returns nothing rather than a near miss.
Where does this data come from?
British Steel's own published datasheets to BS EN 10365:2017, from the manufacturer that rolls them. Every row is reconciled against its own geometry before it is published here: the mass is recomputed from the flanges, web and fillets and has to agree within 3%. One row that would not reconcile is left out rather than guessed.